Bile pigments in plants and animals are structurally related by virtue of their common origin from the carbon skeleton of protoporphyrin IX, but are otherwise distinct in at least two fundamental ways. First, plant bile pigments serve as photoreceptors for a host of light mediated processes necessary for ontogenetic development of the organism, whereas mammalian bile pigments are transient products of hemoglobin catabolism with no known physiological function. Second, plant bile pigments are covalently linked to specific cellular proteins and the resulting chromophore-protein interaction renders them insensitive to photodestruction in vivo. In contrast, mammalian bile pigments form non-covalent bonds with proteins and are susceptable to destruction in high intensity light. Work in progress on this project is aimed at (1) determining the mechanism of bile pigment formation in plants, (2) determining the primary structure of plant proteins to which bile pigments are covalently linked, (3) determining the properties of bile pigment-protein interaction which render plant bile pigment stable in the light.